By their continuous transformations along the day and the year, Sun and sky light are modeling, visually and energetically, the cities and buildings. How this information can be synthesized and used in the project, considering together the random occurrence of the passing clouds and the astronomical regularity of the sun paths?
Regarding to the simulation tools, the main progress in late years has been produced in the render software, that force the users to separate the different components of light in any construction of an illuminated scene, using the opportune algorithms for direct Sun light, sky aperture, diffuse and specular reflections of Sun and sky.
It will be first shown how the best mode of introducing these components into a design logic consists in taking advantage of the geometrical properties of different projections, as the stereographic, equivalent, orthographic, gnomonic or isochronal ones.
It will be also remembered that the apparently simplest problem – Sun paths – conduces the architect to a five dimensions problem. With the example of “Heliodon 2” software, [Beckers & Masset, 2009], it will be shown how to design with the double geometry of Sun paths and sky light.
Due to the growth and necessary densification of our cities, new problems are appearing: the right for everybody to see the sky, the energies economy, the solar energy collect and distribution – cities are then considered as energy sources and not only as consummation places –, the control of the mutual impact between the city and the atmosphere.
With this last theme, we are in front of a “multiscale problem”, as say the physics when a unique problem, as the urban heat island, is the product of innumerable contributions that have to be studied together and simultaneously in so different scales that they are not governed yet by the same equations. We must listen to build architectures thinking on the city, cities thinking on the territory, and to conceive the territory itself from the cities, because our good or bad urban ideas model the most distant countries, and the entire planet.

CitacióBeckers, Benoit. Las escalas de la luz. A: International Conference Virtual City and Territory. "5th International Conference Virtual City and Territory, Barcelona, 2,3 and 4 June 2009". Barcelona: Centre de Política de Sòl i Valoracions, 2009, p. 417-426.